Ground-to-air antennas are designed to emit radiation towards the sky, such as towards airplanes. Ground-to-air antennas may also be used to emit radiation from an elevated position towards the ground, such as in stadiums or indoor applications.
Because of the above, the elevation pattern of such antennas must form a specific shape to provide the required radiation coverage at all angles, up to 90 degrees from the horizontal. Ideally, this elevation pattern takes path loss compensation at each tilt of the antenna into consideration. FIG. 1 shows such an example of an ideal elevation pattern for ground-to-air antennas based on path loss. This pattern may not be ideal for all applications.
FIG. 1a shows a typical base station pattern with mechanical uptilt. Typical base station antennas create elevation patterns with a null signal directly overhead of the antenna due to the effect of each antenna element's pattern. This is mostly due to the positioning of the array at 90 degrees to the horizon which will give almost zero radiation at 90 degrees above the horizon.
One solution to overcome this issue involves mechanically tilting the antenna unit towards the sky. However, mechanical tilting at certain angles results in problematic configurations for tower-mounted antennas, as shown in FIG. 2. These tower-mounted antennas can be difficult to mount, can be subject to high mechanical stresses, and do not provide the coverage desired.
Another known solution to the null signal produced at 90 degrees (i.e. directly above the antenna) is the use of custom-shaped beam elements in place of an array of antennas. FIG. 3 shows an example of a state of the art ground-to-air antenna elevation pattern from U.S. Pat. No. 6,735,438. However, in such configurations, due to wide beamwidth, gain is low and the angle of the maximum beam cannot be modified easily.
There is therefore a need to mitigate, if not overcome, the shortcomings of the prior art.